1. Field of the Invention
The present invention relates to a method for obtaining tridimensional optical image formation of an object by means of irradiations, as well as a device used for the application of this method. A tridimensional reconstruction is effected by the processing of a series of bidimensional measurements of attentuation of radiation through the object, the incidence of radiation being modified between said measurements.
2. Description of the Prior Art
Bidimensional optical image formation from bidimensional measurements of the attenuation of radiation is well-known. The equipment comprises a source of suitable radiations, such as X-rays for medical examinations; the object to be examined is placed between the irradiating source and a sheet of paper or sensitive film whose points have an image produced according to the intensity of the rays at the outlet of the object; the contrasts observed concerning the image indicate the position of the absorbent zones of the object
However, the information thus obtained is inadequate for certain applications and consequently methods for the tridimensional reconstruction of the object have been proposed.
Magnetic resonance tomography methods require the use of costly installations and extremely wide uniformity of the magnetic field where the object to be examined is placed. Moreover, the measuring times required to embody a tridimensional reconstruction are extremely long. Consequently, these drawbacks limit the advantages of these methods.
It has even been proposed to establish tridimensional reconstructions by the superimposition of bidimensional reconstructions or sections of the object. A collimation of an X-ray source makes it possible to obtain a fan-shaped radiation which traverses one section of the object and then produces an image of a line of sensors; the source rotates around the object so as to irradiate the same section under different angles; the successive measurements are stored and a computer makes it possible to determine local contribution on attenuation in each point of the meshing of the section. The source and the sensors are then offset and carried onto another section around which they move according to a trajectory parallel to the previous one.
The examination time thus depends on the adopted number of trajectories. In practice, it is unfortunately not possible to allow for an axial sampling as compacted as the sampling inside a section; the object also risks moving between examinations of the two sections, which increases localization uncertainties.
A further drawback is linked to collimation, which reduces the energy efficiency of the source and which may require that the device be stopped from time to time during the examination so as to allow it to cool down.
Methods have also been proposed which use a conical beam which rotates around the object and by which it is possible to carry out several irradiations which provide many bidirectional images of the object. If there is a sufficient number of these images, a computer can analyze and combine these images in order to reconstitute a tridimensional image of the object. These methods use what is known as the transformed Radon of the attenuation of radiation at each point of the meshing of the object. The transformed Radon of a function at a point is equal to all the sums of the local values of this function on each plane passing through at least one point of the range where the function is delivered. In practice, this clearly satisfies a discrete topology with a finite number of planes in order to describe the transformed Radon and a finite number of points in order to describe the function.
Measurement of attentuation of radiation on a line of sensors of a flat detector placed behind the object provides attenuation according to a beam of rays contained in a plane of the Radon space.
The sum of the attenuation along this line gives the value of the transformed Radon of attenuation for this plane. The numerical inversion of the transformed Radon gives attenuation at all points of the definition range of the function. It needs to be acknowledged that this entire process is complicated and that certain of the methods proposed result in erroneous results being obtained or all results nevertheless being inaccurate.
Amongst the literature available, reference may be made here to the article by Schlindwein and entitled "Iterative three--dimensional reconstruction from twin--cone beam projection" (IEEE Transactions on nuclear science, vol. NS-25, n.degree.4, October 1978, pages 1135-1143), Where the methods using the transformed Radon are rejected owing to their complexity, and that of Minerbo entitled "Convolutional reconstruction from cone--beam projection data" (IEEE Transactions on nuclear science, vol. NS-26, n.degree.2, April 1979, pages 2682-2684) which uses a method implementing the transformed Radon.
As revealed subsequently by the invention, the use of the transformed Radon itself nevertheless requires the use of approximations in the numerical calculations, and moreover the articles of the prior Art do not provide concrete devices allowing for reconstructions of good quality of tridimensional images.